That's a limitation of the Meanwell driver. They clip below 15%...much to my dismay.
As you've discovered, 15% brightness on these lighting-class diodes is still really bright.
The rated current for the driver is 700 mAmps, or that is the current at which they give the lumen output, and the point on the lumens vs current curve called 100%. 15% of 1300 mA is about 200 mA, and I can't keep them going below about 400 mA. Something else must be happening.
I knew these LEDs are very bright, but I relied on the data in the Cree pdf which shows lumens vs LED current, and which goes down to less than 200 mAmps. I have to believe that Cree isn't fudging data, so there must be something else happening.
I doubt it. The drivers are designed to automatically compensate for voltage drop as current drops.
Is this happening on the XPEs as well, and happening consistently on the two different XML strings?
The XPE's are on a fixed 350 mA driver, so they aren't affected. When I had a similar problem with my last LED light, which had closer to the maximum number of LEDs on the driver, I just adjusted the pot in the driver to a higher voltage output, and the problem went away. This light is acting like the other one - the LEDs light for a few seconds, then go off until I increase the current to them. It won't cost anything to try adjusting the voltage, so I plan to do that just in case it helps.
I'm going to be away from Saturday thru next Thursday, so it may be awhile before I get time to play with this.